For example, the driving and the stop of a load such as a power window driving motor or a lamp to be mounted on a vehicle is controlled by adjusting a DC voltage supplied from a battery through the turning-on and off operation of a semiconductor switch such as a MOSFET.
In such the load circuit, in order to protect the load circuit at the time of the occurrence of an overcurrent, a threshold current is set in advance and a protection device which turns off the semiconductor switch when a load current exceeds the threshold current is provided. Further, when a rush current generated immediately after the turning-on of the semiconductor switch reaches the threshold current, the semiconductor switch may be erroneously turned off. Thus, as disclosed in JP-A-11-51983 (Patent Literature 1), for example, it has been proposed to prevent the erroneous turning-off of the semiconductor switch by setting the threshold current to a high current value immediately after the semiconductor switch is turned on.
According to the Patent Literature 1, a shunt resistor is provided in a load circuit, then a voltage generated at the shunt resistor is compared with a threshold voltage (a voltage corresponding to the threshold current) set in advance, and it is determined that the current is the overcurrent when the voltage generated at the shunt resistor is greater than the threshold voltage to thereby turn off the semiconductor switch. Further, the Patent Literature 1 discloses that charge accumulated in a capacitor is added to the threshold voltage immediately after turning on the semiconductor switch to set the threshold voltage to be greater than the usual value to thereby prevent the erroneous turning-off of the semiconductor switch due to the rush current. That is, even when the rush current is generated to increase the voltage generated at the shunt resistor, since the threshold voltage is set to be high immediately after the semiconductor switch is turned on, the voltage generated at the shunt resistor can be prevented from exceeding the threshold voltage, whereby the erroneous turning-off of the semiconductor switch can be prevented.
According to the technique described in the Patent Literature 1, although the erroneous turning-off of the semiconductor switch due to the rush current generated immediately after semiconductor switch is turned on can be prevented. However, there is a problem that the semiconductor switch can not be turned off immediately, when a dead short occurs immediately after the turning-on of the semiconductor switch and the overcurrent flows. That is, since the threshold voltage (threshold current) is set to be higher than the usual threshold value immediately after the semiconductor switch is turned on, it takes a long time to determine the overcurrent. Thus, there arises a problem that circuit components such as electric wires and the semiconductor switch are overheated during the time until the determination of the overcurrent.
Further, since there is a case that a plurality of loads are provided on the downstream side of the semiconductor switch, and the plurality of loads are respectively provided with switches separately. In such the case, even in a case where the semiconductor switch is turned on and the current becomes stable upon the lapse of a certain time, a rush current flows again when the switch provided on the downstream side is turned on. Thus, there arises a problem that the semiconductor switch is erroneously turned off when the rush current exceeds the threshold current.